The digestive system provides receiving, mechanical and chemical processing of food, products absorption of splitting and removal of undigested residues.

The organs of the digestive system include digestive channel through which food passes (mouth, esophagus, stomach, intestines) and digestive glands (salivary, pancreas, liver, etc.). The wall of the digestive tract consists of three layers: the outer, middle and inner. The outer layer is formed by connective tissue that separates the digestive tube from surrounding tissues and organs. The middle layer - muscular. In the upper sections muscle layer is formed by striated and in the middle of the esophagus - by smooth muscle tissue. The inner layer - mucosa.

The anus, or anal canal, is the final segment of the gastrointestinal tract. It acts as the orifice that feces pass through during defecation.

Anatomy of the anus

The anus is a short tube at the end of the rectum that ends at the body’s exterior. It is around 1 inch (2-3 cm) long and varies widely in its diameter depending on how distended it is. The superior end of the anus is continuous with the tissue of the rectum and is lined with simple columnar epithelial tissue. This tissue forms folds known as anal columns, with valleys known as anal valves between the folds.

The rectum is the final segment of the large intestine that connects the colon to the anus. It stores fecal matter produced in the colon until the body is ready to eliminate the waste through the process of defecation.

Anatomy of the rectum

The rectum is a hollow muscular tube about 8 inches (20 cm) in length and 2½ inches in diameter at its widest point. It extends from the inferior end of the sigmoid colon along the anterior surface of the sacrum and coccyx in the posterior of the pelvic cavity. At its inferior end, the rectum tapers slightly before ending at the anus.

The sigmoid colon is a curved, S-shaped region of the large intestine and is the final segment of the colon. It transports fecal matter from the descending colon to the rectum and anus. Feces are stored in the sigmoid colon until they are ready to be eliminated from the body through the anal canal. The intestinal wall of the sigmoid colon also plays a small role in the absorption of water, nutrients and vitamins from feces.

The sigmoid colon is a hollow tube approximately 2 ½ inches (7 cm) in diameter and 16 inches (40 cm) long. A band of smooth muscle tissue running the length of the colon contracts to form many small pouches called haustra in the walls of the sigmoid colon. Haustra help to increase the surface area of the colon, while their smooth muscle tissue helps to move feces toward the end of the colon. At its superior end, the sigmoid colon connects to the descending colon near the iliac crest in the left iliac region of the abdominal cavity. From this junction, the sigmoid colon curves medially and anteriorly toward the body’s midline, before abruptly turning about 90 degrees to face inferiorly to form the rectum.

The descending colon is a segment of the large intestine and is the third and penultimate segment of the colon. It transports feces from the transverse colon inferiorly along the left side of the abdominal cavity to the sigmoid colon. Feces passing through the descending colon are stored until they are ready to be eliminated from the body. The walls of the descending colon absorb water as well as remaining nutrients and vitamins from the feces, depositing these valuable substances into our bloodstream.

The descending colon is a hollow tube that is part of the gastrointestinal (GI) tract. Its diameter is roughly 2.5 inches (7 cm), while its length is approximately 9 to 10 inches (25 cm). It contains many small pouches, known as haustra, along its length that increase its surface area and help to move feces through the colon. At its superior end, the descending colon connects to the transverse colon at the splenic flexure just inferior to the spleen. From the splenic flexure, the descending colon extends inferiorly toward the left hip before turning about 90 degrees to the right and forming the sigmoid colon.

The transverse colon is the longest region of the colon and is located between the ascending colon and descending colon. It is named for the fact that it crosses the abdominal cavity transversely from the right side to left side just below the stomach. Much of the absorption and feces formation of the colon takes place in the transverse colon, making it a very important region of the digestive system.

Anatomy of the transverse colon

The colon is the main portion of the large intestine and is divided into four regions: the ascending, transverse, descending, and sigmoid colon. The transverse colon begins at the hepatic flexure, a sharp bend at the superior end of the ascending colon just inferior to the liver on the right side of the abdominal cavity. At the hepatic flexure, the colon turns to the left and crosses to the left side of the abdominal cavity as the transverse colon. Upon reaching the left side, the colon again turns sharply toward the inferior direction at the splenic flexure before continuing on as the descending colon.

The ascending colon is one of the four major regions of the colon, which is itself one of the parts of our large intestine. The ascending colon carries feces from the cecum superiorly along the right side of our abdominal cavity to the transverse colon. In the ascending colon, bacteria digest the transitory fecal matter in order to release vitamins. The intestinal wall absorbs water, nutrients, and vitamins from the feces and deposits these materials into our bloodstream.

Extending from the inferior end of the large intestine’s cecum, the human appendix is a narrow pouch of tissue whose resemblance to a worm inspired its alternate name, vermiform (worm-like) appendix. It is located in the right iliac region of the abdomen (in the lower right-hand abdominal area), measuring about four inches long and roughly a quarter of an inch in diameter.

Like the rest of the digestive tract, the appendix is made of an inner layer of mucosa with submucosa, muscularis, and serosa layers surrounding it. Unlike the rest of the large intestine, however, the submucosa of the appendix contains many masses of lymphoid tissue. The presence of lymphoid tissue suggests that the appendix may play a role in the immune system in addition to the digestive system.

The cecum is the cul-de-sac at the beginning of the large intestine that descends from the union of the ileum and the large intestine. It provides a space for the mixing of bacteria with partially digested food from the small intestine to form feces.

Anatomy of the cecum

The cecum is a short, pouch-like region of the large intestine between the ascending colon and vermiform appendix. It is located in the lower right quadrant of the abdominal cavity inferior and lateral to the ileum.

The large intestine is the final section of the gastrointestinal tract that performs the vital task of absorbing water and vitamins while converting digested food into feces. Although shorter than the small intestine in length, the large intestine is considerably thicker in diameter, thus giving it its name. The large intestine is about 5 feet (1.5 m) in length and 2.5 inches (6-7 cm) in diameter in the living body, but becomes much larger postmortem as the smooth muscle tissue of the intestinal wall relaxes.

The large intestine wraps around the border of the abdominal body cavity from the right side of the body, across the top of the abdomen, and finally down the left side. Beginning on the right side of the abdomen, the large intestine is connected to the ilium of the small intestine via the ileocecal sphincter. From the ileocecal sphincter, the large intestine forms a sideways “T,” extending both superiorly and inferiorly. The inferior region of the large intestine forms a short dead-end segment known as the cecum that terminates in the vermiform appendix. The superior region forms a hollow tube known as the ascending colon that climbs along the right side of the abdomen. Just inferior to the diaphragm, the ascending colon turns about 90 degrees toward the middle of the body at the hepatic flexure and continues across the abdomen as the transverse colon. At the left side of the abdomen, the transverse colon turns about 90 degrees at the splenic flexure and runs down the left side of the abdomen as the descending colon. At the end of the descending colon, the large intestine bends slightly medially at the sigmoid flexure to form the S-shaped sigmoid colon before straightening into the rectum. The rectum is the enlarged final segment of the large intestine that terminates at the anus.